Electrically-operated injection molding machines are provided with various operating units including injection units and the like, each of which is driven by one or more servomotors, so as to perform injection molding cycles including an injection process, etc. In each respective process of the injection molding cycle, positioning control for one or more control axes, corresponding to the process, is carried out. For instance, in the final stage of the injection process, positioning control to an injection end position on an injection axis is carried out. To this end, injection molding machines of this kind are provided with servo circuits for individual axes so as to control the drive of the servomotors for these axes. Each of the servo circuits receives a pulse train, indicative of a target motor rotation position and supplied from a numerical control unit, and another pulse train, indicative of an actual motor rotation position and supplied from a position detector mounted on the servomotor concerned, to effect positioning to the target rotation position.
Generally, in order to effect the positioning with a desired accuracy and within a desired setting time, the positioning is completed when the actual motor rotation position falls within an allowable positioning zones having a predetermined width and consisting of positive-side and negative-side regions, which are adjacent to each other with respect to the boundary therebetween where the target rotation position is located. Here, the positive-side region is the portion of the allowable positioning zone which is at the side of the target position that is away from the origin of the coordinate system, which is set for the associated axis for the target position, and the negative-side region is the portion of the zone which is at the side of the target position that is toward the coordinate origin. Conventionally, the width of the positive-side region of the allowable positioning zone and the width of the negative-side region thereof are set to the same value of "W/2", as shown in FIG. 4, wherein the reference symbol W indicates the total width of the allowable positioning zone, and PS indicates the target position.
In certain cases, it is preferable to effect the positioning for each of the control axes in a specific manner which depends on an operating mode of the associated operating unit such as the direction to which the operating unit is operated and the function to be achieved by the operating unit. For instance, in order to effect an injection by which a predetermined amount of shot can be attained, preferably, the injection process should be completed and a shift to the hold process should be made at a screw-moving-position which slightly exceeds the target injection end position. On the other hand, in order to protect a mold of a type which is liable to be damaged when a mold-opening operation is continued even after a predetermined mold-opening stroke is reached, the mold-opening process should preferably be completed at a movable-platen-moving-position that is slightly short of the target mold-opening end position. In other words, if the respective origins of the coordinate systems set for the injection and the mold-clamping axes are located at sides opposite to the directions of injection and mold-clamping, the injection screw should preferably be positioned at a position falling within the negative-side region of the allowable positioning zone associated with the target injection end position, whereas the movable platen should preferably be positioned at a position falling within the positive-side region of the allowable positioning zone associated with the target mold-opening end position.
However, the aforementioned conventional method cannot achieve the positioning of a unit of the machine to a position which arbitrarily is within or the other one of the positive-side and negative-side regions of the allowable positioning zone, although the positioning within this zone can be surely made. In this respect, conventionally, the target position is shifted to either the positive or negative side a distance equal to the half width "W/2" of the allowable positioning zone, where required to accommodate the operating manner of an associated operating unit of the injection molding machine. More specifically, if positioning to a moving position exceeding the target position PS should be made, as with the aforesaid case for shifting from the injection process to the hold process, the target position is set to "PS+W/2", as shown in FIG. 5. On the other hand, if positioning to a moving position short of the target position should be made, as in the case of mold-opening end operation, the target position is set to "PS-W/2", as shown in FIG. 6. In this manner, the respective target positions are shifted, if necessary, to accommodate the operating manner of the associated operating unit. Accordingly, labor is required for preparing the control program. Moreover, positioning accuracy is diminished because the effective width of a corresponding one of the positive-side and negative-side regions of the allowable positioning zone is increased due to the shift of the target position.